1. Field of the Invention
This invention relates to a power MOSFET, IGBT among power semiconductor devices constructing a power converting device such as an inverter and more particularly to a device structure which causes a characteristic of resistance to load short circuit to be enhanced.
2. Description of the Related Art
In order to meet requirements for miniaturizing a power supply device and making the same to have highly sophisticated functions in the recent power electronics field, much attention is paid to the improvement of the characteristics of a power semiconductor device such as the high withstand voltage characteristic, large current characteristic, low loss characteristic, high breakdown voltage characteristic and high speed characteristic. Particularly, from the viewpoint of the high withstand voltage characteristic and large current characteristic, an IGBT which can attain lower turn-ON voltage than a power MOSFET is used as a power semiconductor device having a withstand voltage of approx. 300V or more.
As this type of IGBT, two types of devices, that is, a planar structure in which gates are provided in a flat form and a trench structure in which gates are buried in grooves are widely known.
FIG. 11 is a cross sectional view showing the structure of an IGBT having the planar structure. In the IGBT, a p-type collector layer 102 of high impurity concentration is formed on one surface of an n-type base layer 101 of high resistance, a p-type base layer 106 is selectively formed on the other surface thereof, and an n-type emitter layer 107 is selectively formed on the p-type base layer 106.
A gate oxide film 103 is formed on the n-type base layer 101, part of the p-type base layer 106 and part of the n-type emitter layer 107. Further, an emitter electrode 109 is formed on the p-type base layer 106 and n-type emitter layer 107. A gate electrode 105 is electrically isolated from the emitter electrode 109, n-type base layer 101 and p-type base layer 106 by the presence of the gate oxide film 103 and an interlaid insulating film 104.
An electron injecting MOSFET having a channel region CH is constructed by the n-type base layer 101, p-type base layer 106, n-type emitter layer 107 and gate electrode 105. Further, the emitter electrode 109 is formed on the p-type base layer 106 and n-type emitter layer 107 so as to make contact with both of the layers.
In the IGBT with the conventional planar structure described above, since the n-type emitter layer 107 and p-type base layer 106 are formed by ion implantation of impurities and thermal diffusion thereof, the impurity distributions thereof will correspond to the Gaussian distribution. Therefore, when attention is paid to the impurity concentration distribution along the channel region CH, it is understood that the maximum value of the p-type impurity concentration lies in a position near the junction between the n-type emitter layer 107 and the p-type base layer 106 and the p-type impurity concentration gradually becomes lower in a portion closer to the junction between the p-type base layer 106 and the n-type base layer 101.
Next, the operation of the IGBT is explained. Since the operation principle is substantially the same in the IGBT with the planar structure and the IGBT with the trench structure, the operation of the IGBT with the planar structure is explained below.
If positive voltage which is positive with respect to voltage applied to the emitter electrode 109 is applied to the gate electrode 105 when positive voltage is applied to a collector electrode 108 and negative voltage is applied to the emitter electrode 109, then the surface of the p-type base layer 106 which faces the gate electrode 105 is inverted to an n type and electrons e are injected from the n-type emitter layer 107 into the n-type base layer 101 via the inverted layer and reach the collector layer 102. As a result, the n-type base layer 101 and p-type collector layer 102 are forwardly biased so as to cause holes h to be injected from the p-type collector layer 102 into the n-type base layer 101. Thus, both of the electrons e and holes h are injected into the n-type base layer 101 and conductivity modulation occurs in a region of the n-type base layer 101 to lower the turn-ON voltage. That is, the device is set into the conductive state.
On the other hand, in order to turn OFF the device, voltage which is negative with respect to voltage applied to the emitter electrode 109 is applied to the gate electrode 105. As a result, the inverted layer formed in the surface portion of the p-type base layer 106 which faces the gate electrode 105 disappears and electron injection is interrupted. At this time, part of the holes h stored in the n-type base layer 101 is discharged to the emitter electrode 109 via the p-type base layer 106 and the remaining holes are recombined with electrons e and disappear and thus the device is turned OFF.
If the device is set into a load short circuit condition, the device is set in the conductive state and the power supply voltage is applied to the collector electrode 108. Therefore, a large short circuit peak current (Icp) flows in the device and the device is destroyed after a preset period of time (tsc). In the conventional IGBT, the turn-ON voltage can be lowered by increasing the channel density, but at this time, an increase in the channel density causes a current to easily flow. As a result, there occurs a problem that the short circuit peak current (Icp) becomes larger and the characteristic of resistance to load short circuit (tsc) is degraded.
As described above, in the conventional semiconductor device, a problem that the sufficient characteristic of resistance to load short circuit cannot be attained occurs.